This application relates to a ground fault interrupt that is subject to fewer nuisance trips than existing circuits.
Ground fault interrupt circuits are provided to stop the flow of current if an undesirable condition is sensed. They are typically utilized in applications where there is a likelihood that a human can be injured if the undesirable current condition is allowed to occur. When utilized in most common electrical applications, the provision is relatively straightforward. There are challenges to providing ground fault interrupt above 1 kHz, as with such frequencies, there is a significant current that is not returned to a neutral wire through a ground fault isolator, but instead returns to ground structures. These frequencies need to be excluded from the ground fault measurement, as they might otherwise cause unnecessary shutdowns. On most ground-based systems, the fundamental frequency of AC power is fairly low, and on the order of 50 or 60 Hz. Thus, it is easy to implement a filter that has no significant attenuation for the fundamental frequency (50 or 60 Hz) but good attenuation for harmonics that are greater than 1 kHz.
However, on aircraft, AC power sources run typically at 400 Hz, with a maximum range up to 800 Hz and even higher. Therefore, implementing a filter with no attenuation at the fundamental frequency, 800 Hz for example, and sufficient attenuation at frequencies above 1 kHz is much more challenging.
This becomes particularly difficult when the system that is provided with the ground fault interrupt is one that might have human contact. As an example, the power supply for an entertainment system in an aircraft passenger seat raises challenges. The necessary low ground fault threshold (which may be as low as 20 mA) adds to the problem. Harmonics from such loads (e.g. switch mode power supplies) can be higher than the ground fault detection threshold.
Present ground fault protectors do not provide enough sensitivity to distinguish between fundamental and harmonic frequency content, and therefore result in nuisance trips, especially when configured for the low current level thresholds associated with systems to which humans may be exposed. Nuisance trips will often require that an operator manually reset a breaker or other switch. This can be time consuming and potentially dangerous if an actual ground fault condition does exist. Thus, reducing the number of nuisance trips would be beneficial.